Hydraulic fluid circuits supply fluid pressure to various fluidic components, such as valves, cylinders, and pistons or other actuators in a vehicle transmission. Primary hydraulic pressure is typically provided via a fluid pump. During periods in which the fluid pump cannot provide all of the required hydraulic pressure for a given task, supplemental hydraulic pressure may be provided via a hydraulic accumulator. The hydraulic accumulator thus acts as a standby energy storage device in such a fluid circuit.
In order to charge the accumulator with hydraulic fluid pressure, a valve is selectively opened when the fluid pump is running. Opening of the valve allows hydraulic fluid pressure to act on a piston located within the hydraulic accumulator. The resultant movement of the accumulator piston compresses a volume of nitrogen or another inert charging gas within the hydraulic accumulator. As the charging gas is compressed, hydraulic fluid enters and fills the hydraulic accumulator where the hydraulic fluid is then stored under pressure for later use. The minimal threshold volume of hydraulic fluid required for commanding a refilling of the hydraulic accumulator is referred to in the art as the recharge or reserve volume.